Photocurable prepolymers and compositions are well known in the art for forming printing plates and other radiation sensitive articles. In the field of flexographic printing plates, the plates typically comprise a support and a photosensitive layer prepared from the photocurable composition. Additional layers on the plate include slip and release films to protect the photosensitive surface. Prior to processing the plate, the additional layers are removed, and the photosensitive layer is exposed to radiation in an imagewise fashion. The unexposed areas of the layer are then removed in developer baths.
Removal of unexposed layers comprising photocurable compositions such as those disclosed in U.S. Pat. No. 2,760,863 require the use of developer baths comprising environmentally unsafe, organic solvents such as tetrachloroethylene, perchloroethylene, 2-butanone, benzene, toluene, xylene, trichloroethane and solvent mixtures such as tetrachloroethylene/n-butanol. However, due to the toxicity, high volatility and low flash point, their use gives rise to hazardous conditions and creates pollution problems. Thus, recently there has been a strong interest in the field to develop photosensitive layers in non-organic solvent developing solutions, e.g. aqueous or alkaline-aqueous solutions. However, the compositions resulting from recent attempts to achieve aqueous developable plates demonstrate deficiencies in mechanical properties or are unsuitable for use with a wide variety of printing inks, especially alcohol based inks.
For instance, in addition to possessing an aqueous developable photosensitive layer, a flexographic printing plate must have sufficient flexibility to wrap around a printing cylinder, yet be strong enough to withstand the rigors experienced during typical printing processes. Further, the printing plate should be soft enough to facilitate ink transfer during printing.
Previous aqueous developable compositions have not possessed all the desirable features such as flexibility, softness and solvent resistance to inks typically used in printing. For example, water-developable photosensitive compositions which contain as the main component a high molecular weight polymer such as polyvinyl alcohol, cellulose, polyethylene oxide, or the like, are insufficient in flexibility and rubber hardness and hence are unsuitable for use in flexographic printing plates.
Finally, it is also important that the photosensitive layer of the printing plate be dimensionally stable during storage. For example, some compositions used for making plates have shown inferior stability properties when used in solid flexographic printing plates in that the compositions become tacky and pasty during storage. Those inferior properties have been attributed to the low molecular weight of the polymers used to prepare the printing plates. See Koch et al. U.S. Pat. No. 4,762,892 infra and discussion of low molecular weight polymers disclosed in Japanese Kokoku 57-23693.
One attempt to develop an aqueous developable, flexible printing plate is illustrated in Koch et al. U.S. Pat. No. 4,916,045. The patent describes a photosensitive recording element consisting primarily of an ionic polymer. The ionic polymer is prepared by the partial esterification or amidation of succinic acid anhydride side groups on an alkadiene polymer. The resultant polymer is then converted into an ionic polymer by reaction with suitable compounds. With reference to the preferred materials, the reference is directed to oligomers having a polybutadiene backbone, side radicals prepared by (a) maleating the polybutadiene, (b) esterifying the resultant pendent succinic anhydride ring with hydroxyethylmethacrylate (HEMA), and (c) neutralizing the carboxyl groups generated during the esterification with a cation from Groups I to V of the Periodic Table. The polymer is not chain-extended with, e.g., polybutadiene diol, or reacted (toughened) with, e.g., monohydroxy-terminated polystyrene.
Imaizumi et al., U.S. Pat. No. 4,023,973, Photosensitive Composition Using Maleic Anhydride adduct of a 1,2 Polybutadiene (1977), discloses photosensitive compositions for use on flexographic printing plates developable in water. The compositions comprise the maleic anhydride adduct of 1,2 polybutadiene, photosensitizer, and acrylate extenders. The adduct may be hydrolized to open the anhydride rings, and the product may be neutralized with a base. The adduct is not chain extended, nor is any styrene polymer used. The 1,2- content of the material, 70% or more, gives the composition an undesirably high rubber hardness.
Stephen Proskow U.S. Pat. No. 4,272,608, Photosensitive Compositions Containing Thermoplastic Ionomeric Elastomers Useful in Flexographic Printing Plates (1981) discloses a photosensitive composition useful in flexographic printing plates. The composition comprises a maleated butadiene-acrylonitrile copolymer which can be reacted with a (meth)acrylate. The plate can be developed in water containing sodium carbonate.
Fickes et al. U.S. Pat. No. 4,478,931, Precured Flexographic Printing Plate (1984), discloses a photopolymer prepared by mixing a carboxyl-containing butadiene-acrylonitrile block copolymer with an acrylate. The carboxyl groups may come from maleic anhydride. Hydroxy-alkyl(meth)acrylates are not disclosed. The butadiene may contain styrene groups in the form of a block copolymer, not reacted with maleic groups as in the instant invention. The polymer is not chain-extended. The photopolymer may be used in a water-developable printing plate.
Other attempts to make aqueous developable printing plates include:
Rousseau et al. U.S. Pat. No. 4,507,382, Water Developable Positive Acting Lithographic Printing Plate (1985) discloses a water developable lithographic printing plate using a photopolymerizable composition comprising an ethylenically unsaturated dextrin oligomer and a hydroxy acrylamide.
J. R. Worns U.S. Pat. No. 4,517,279, Photosensitive Elastomeric Polymer Composition for Flexographic Printing Plates Processable in Semi-Aqueous Basic Solution or Solvent Systems (1985), discloses a photosensitive composition comprising a mixture of carboxylated and non-carboxylated butadiene/acrylonitrile copolymers. Plates made with the composition were developed in semi-aqueous, e.g., alkanol/water solutions.
J. Fujikawa U.S. Pat. No. 4,621,044, Photosensitive Polymer Composition(1986), discloses a water developable printing Plate using polyvinylacetate as base polymer.
Lori J. Klinger U.S. Pat. No. 4,857,434, Radiation Curable Liquid (Meth)Acrylated Polymeric Hydrocarbon Maleate Prepolymer and Formulations Containing Same (1989), discloses methacrylation of a maleated hydrocarbon, e.g., polybutadiene for the preparation of a photopolymer useful in water-developable flexographic printing plates.
Nebe et al. U.S. Pat. No. 5,047,313, Photosensitive Semi-aqueous Developable Copper Conductor Composition (1991) discloses a flexographic printing plate requiring some organic solvent (e.g., butyl cellosolve) for development. The plate has a photosensitive component comprising glass frit, acrylate binder, and acrylate monomer.